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International audience; This study aimed at evaluating potential arsenic (As) mobility in an industrially contaminatedsoil (64 mg As kg-1) of the Meuse River basin, and at identifying key bacterial groups that drive soil As dynamics. Both speciation and release of As from this soil was followed under anaerobicconditions using a laboratory batch experiment. In the presence of exogenous carbon sources,AsV initially present in the soil matrix and/or adsorbed on synthetic hydrous ferric oxides wassolubilized and mainly reduced into AsIII by indigenous soil microfora. After a one-monthincubation period in these biotic conditions, AsIII accounted for 80-85% of the total dissolved Asand more than 60% of the solid-phase As. Bacterial community structure (i.e. 16S rDNA-basedCE-SSCP profiles) changed with incubation time and As amendment. The detection of distantlyrelated arsenate respiratory reductase genes (arrA), as functional markers of AsV-respirers,indicates that novel dissimilatory AsV-reducing bacteria may be involved in Asbiotransformation and mobility in anoxic soils. Since As and iron were concomitantly released, acrucial role of indirect As-mobilizing bacteria on As behavior was also revealed. Our results show that the majority of As within the soil matrix was bioavailable and bioaccessible forheterotrophic AsV reduction to AsIII, which may increase As toxicity and mobility in thecontaminated soils.

International audience; Variations in the stable-isotope compositions of O and H in a catchment's water balance are mainly caused by natural variations in the isotopic composition of rainfall, through mixing with pre-existing waters and evaporation. Stable isotopes were analysed in surface waters along the main course of the Ebro River (Spain), in its main tributaries and in some groundwaters as part of a year-long monthly survey at the outlet. Meteoric input data over the basin from five IAEA/GNIP stations surrounding the Ebro catchment showed that the mean weighted δ18O–δ2H rain input showing enriched values for four stations and a depleted value for the fifth. Most surface- and ground-waters collected in the Ebro catchment plot close to the global and local meteoric-water lines, reflecting a meteoric origin and a lack of significant evaporation or oxygen-isotope exchanges between water and the rock matrix. Although the main tributaries show large variations of δ18O and δ2H signatures (respectively − 5.0 to − 9.6‰ and − 38.5 to − 63.6‰) only the Guadalope river shows an evaporated signal. All tributaries have more depleted δ18O and δ2H values than the mean rainwater δ18O and δ2H signal for the Ebro catchment given by local rain monitoring stations. The depleted values in the Ebro tributaries originate from runoff from the Pyrenees, whereas normal Mediterranean rains yield mostly enriched values. The Ebro River itself shows δ18O and δ2H values that fall between those of its tributaries and no variation is related to the location along the river course. The river water at the outlet (Amposta) display a range of δ18O–δ2H values from respectively − 7.2 to − 8.4‰ and − 47.5 to − 53.9‰, though with a few enriched values. There is no link between river discharge and Cl content. Temporal δ18O–δ2H variations at the Ebro outlet reflect input of enriched water from Mediterranean rains and of depleted waters from Pyrenean runoff. The geochemical signal at the Ebro basin outlet was deconvolved through a conceptual scheme of the Ebro hydro system, highlighting the major role played by dam impoundments, not only on water quantity, but also on its quality.